The present invention relates to variable displacement hydrostatic pumps of the axial piston type, and more particularly, to such pumps of the type wherein the pump displacement is controlled by a fluid pressure actuated servo assembly.
Many variable displacement hydrostatic pumps of the type to which the present invention relates are utilized in the field of mobile hydraulics, i.e., as part of the hydraulic system of various types of vehicles. Furthermore, many such pumps are utilized in the "propel circuit", i.e., to supply fluid under pressure to a fluid motor which transmits torque to the drivewheels of the vehicle.
An example of a vehicle which would typically utilize a hydrostatic propel circuit would be a skid-steer loader. It is desirable to provide the axial piston pump with a fluid pressure-actuated servo assembly to control the pump displacement, rather than requiring the vehicle operator to control pump displacement manually, in order to minimize operator fatigue. One of the key performance criteria for a servo assembly of the type used to vary the displacement o an axial piston pump is the responsiveness of the servo. In other words, as the pump is displaced from its neutral position (zero output flow) to a particular displaced position (either forward or reverse) it is not acceptable to have excessive time delay between movement o the manual control by the operator and changes in the pump displacement. The rate of change of pump displacement, for a given movement of the manual input, is referred to as the "gain rate".
It has generally been considered desirable by the operators of such vehicles to have a relatively high gain rate when selecting relatively large pump displacements. However, it has been found that on vehicles having a high-gain rate pump and servo assembly, the attempt by the operator to make small corrections in the pump displacement typically results in excessive displacement changes, making it difficult to control the vehicle, especially when maneuvering the vehicle in tight quarters.